Once you accept someone’s apology, stop bringing it up. Using it as ammo later cancels the forgiveness.

Forgiveness and a stored grievance cannot both be true. Accepting an apology and then producing it in the next argument means the first part was a loan rather than a settlement, and the other person learns that saying sorry to you creates a weapon instead of ending a problem. That is why people stop apologising properly. Being honest with you got too expensive.

Decide before you accept rather than after: if you are not ready, say you need more time instead of saying it is fine. Once you do accept, retire the thing. If it keeps surfacing, the issue was probably never settled.